1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly to an image forming apparatus which is capable of changing process line speeds for image forming and image transfer operations in accordance with image forming conditions.
This invention also relates to an image forming apparatus which is capable of changing a process line speed of a photoconductor in accordance with user desires for speed and quality in image forming.
2. Discussion of the Background
In an electrophotographic image forming apparatus, there have been proposed several methods for changing the image resolution with a single image forming mechanism. In one method, the image resolution is changed by varying a writing density with a laser beam and a rotation speed of a photoconductive drum while maintaining a rotation speed of a polygon motor unchanged. The image resolution can be changed also by changing a rotation speed of the polygon motor. Specifically, a relatively high image resolution can be obtained by a relatively high speed rotation of the polygon motor. However, an increase of the rotation speed of the polygon motor may require a great deal of structural modification with respect to the polygon motor, causing a relatively high cost increase. Therefore, the rotation speed of the polygon motor is generally controlled to be unchanged.
For example, Japanese Open-Laid Patent Publication No. JPAP01-224780 (1989) describes a technique which varies a density of data writing with a laser beam in accordance with the image resolution and also varies a process line speed in accordance with a change of the density. Further, this technique varies, in accordance with a change of the process line speed, conditions for various factors such as a latent image forming, development, image transfer, image fixing, and so forth.
On the other hand, several methods have been provided for changing a first process speed which is used in an image forming operation and a second process speed which is used in image transfer and fixing operations. One method is provided to avoid an insufficient image fixing operation. More specifically, in an image forming apparatus which uses an intermediate image transfer member, different color images are overlaid one after anther on an intermediate image transfer member in synchronism with a predetermined register mark provided to the intermediate image transfer member. After a completion of such an overlaying operation, the intermediate image transfer member transfers an entire image formed by overlaid images onto a recording sheet. In this image forming operation, if a non-ordinary sheet such as a relatively thick sheet, an OHP (Overhead Projector) sheet, or the like is used as a recording sheet, heat is absorbed by the thick recording sheet and the image transfer efficiency may therefore be reduced. In order to avoid this insufficient fixing operation, the above-mentioned method changes the second process line speed.
For example, Japanese Laid-Open Patent Publication No. JPAP04-67174 (1992) describes a technique which generates a full-color image using a compact fixing unit and with superior quality by reducing a rotation speed of an intermediate image transfer member to an approximate rotation speed of a fixing unit after a full-color and full-one-page image has been completely transferred onto the intermediate image transfer member.
For another example, Japanese Laid-Open Patent Publication No. JPAP06-11977 (1994) describes a technique which relates to process line speeds of the image forming and image transfer operations.
However, none of these techniques comprehensively addresses the problems of image resolution and fixing efficiency, and none necessarily improves the print efficiency in an appropriate manner in accordance with various factors required for performing the image forming operation.
In addition, it is known that when the process line speed with respect to the photoconductive drum and the development mechanisms is changed, development agents on a development sleeve in contact with the photoconductive drum cause an abnormal print on the background of a recording sheet.
An image forming apparatus using an electrophotographic method has also been popularly used, in which an image forming is achieved by a sequential operation including an image exposure on a photoconductor with a laser beam, an image development with toner, and so forth. In such an image forming apparatus, a recording density of image has been desired to be increased. One method is a reduction of a rotation speed, or a process line speed, of the photoconductor, which method however adversely affects operation time. Accordingly, it would be useful if the user can select a process line speed out of a plurality of prestored process line speeds in accordance with a type of job that the user conducts, such as a speed-prioritized job or a quality-prioritized job, for example.
For example, Japanese Laid-Open Patent Publication No. JPAP07-13406 describes a technique which allows the image forming operations at different process line speeds with a measure for avoiding the variations of voltage on the surface of the photoconductor by controlling a grid voltage.
However, controlling the grid voltage is not sufficient to control the stability of the voltage on the surface of the photoconductor which determines the recording density of image, since the voltage on the surface of the photoconductor is determined by amounts of charge supplied by a charging member and light exposure supplied by an optical writing system. That is, when the process line speed is changed, a current value affecting a unit area of the surface of the photoconductor will be varied. Also, a charge level at an image developing position by an image development member will be varied during a time period that the photoconductor rotates so that a position of the surface thereof moves from the charge position to the image development position or that the photoconductor rotates so that a position of the surface thereof moves from the exposure position to the image development position.
As a typical result of the above-described phenomena with respect to the insufficient control of the voltage on the surface of the photoconductor, an image is reproduced on a recording sheet in an inferior condition, having abnormal features such as dirty black stripes and the like.
Therefore, it is understood that there is no image forming apparatus available which is capable of performing an image forming operation in a plurality of different process line speed modes of a photoconductor while avoiding a reproduction of image in an inferior condition.